Polypropylene (PP) films are coated on raffia fabric in industrial extrusion film casting/coating processes. Casting of film is also the first step in a BOPP process. PP, a linear polymer, exhibits shear thinning and extensional thinning properties. Consequently, PP displays significant reduction in the width of the final film/coating in an extrusion film casting or coating process. This is called the ‘necking’ defect. As a result, the processor is left with a film/coating having significantly reduced width as compared to the original die width. Necking also causes the film to thicken at the edges, which is called ‘edge-beading’. In industrial casting processes film edges are typically trimmed, which further reduces the usable width of the film. Moreover, the uneven thickness leads to differential shrinkage resulting in delamination.
The problem/need is to reduce the extent of necking and the consequent edge-beading of extruded film/coating for polypropylene.
In industry, processors run cast film/coating lines at reduced speed to reduce the necking and edge-beading extent. Also, a small amount (typically, 10-20%) of low-density polyethylene (LDPE) is often used as an additive in the base PP polymer to reduce necking. However, the blend of PP and LDPE is an inhomogeneous blend and may lead to other disadvantages such as reduced strength, increased haze and other such similar problems.
US Patent US2010/0087602 to Li et al. discloses a method comprising contacting a polypropylene, an acrylate-containing compound, and an initiator to form a composition, and reactively extruding the composition to form a polymer formulation. This reactive extruded composition is formed into a film where the composition has a melt flow rate that is reduced by equal to or greater than 5% when compared to neat polypropylene and a pressure drop that is lower than that for the neat polypropylene.
US 2012/20730 discloses the high melt strength polypropylene (HMS-PP) functionalized maleated polypropylene. The nanolayer enhancement of biaxially oriented polypropylene film for increased gas barrier is disclosed in Polymer 51 (24), 2010, 5807-5814 by Yijian Lin et al. Further CN101381491 relates to a method for preparing a polypropylene compound with high beta crystal content, comprising of polypropylene resin, an antioxidant and an amide nucleating agent.
While compositions with PP and multi functional acrylate monomers are disclosed in prior arts and possess several properties of industrial application, it is also well known that such combinations cause formation of undesirable side products, such as homo polymers of acrylates. Such side products are known to reduce the purity of the desired polymer product.
In view of above prior arts, the constraint regarding the necking and subsequent edge-beading is not addressed, which is an important industrial problem, affecting the quality of film produced in the casting process.
Further, it may be desirable to have a polymeric composition that is devoid of acrylates and related compounds and polymers such that the PP polymer is presented in a pure form, possibly enhancing its industrial applicability.
Therefore there exists a need in prior art to seek a solution to resolve the problems associated with the extrusion film casting or coating processes.
While prior art processes and compositions have tried this, they usually include acrylates as one of the ingredients, which can lead to formation of unnecessary, undesired related compounds, which affect the purity and therefore performance of the polymer.